Ultrasonic transducers of this type are provided for measuring air flows; however, it is also possible, in principle, to use other fluid media, i.e., gases and/or liquids. Examples of ultrasonic transducers are described in German Patent Application No. DE 10 2007 010 500 A1 as well as in the references cited therein. In principle, the ultrasonic transducers described therein may also be modified and/or manufactured according to the present invention within the scope of the present invention. Ultrasonic transducers of the type described are provided, in particular, for measuring air flows, for example to derive an air volume signal within a system controller of an internal combustion engine.
As is described, for example, in German Patent Application No. DE 10 2007 010 500 A1, matching layers or matching bodies are used in many ultrasonic transducers to improve a noise radiation into the fluid medium and/or an absorption of ultrasound from the fluid medium. For example, ultrasonic transducers having noise-radiating resonance or matching bodies may have, for example, a metal diaphragm and/or a λ/4 impedance matching layer.
In ultrasonic transducers or ultrasonic flowmeters which must meet special pressure requirements, the noise-radiating surface is usually an integral part of a transducer housing or a flow tube, or it is hard-connected to these components, so that it is not usually possible to provide adequate decoupling against the propagation of structure-borne noise. If the decoupling against such noise is to be improved, a soft fastening material is needed. Due to the pressure requirements, however, this fastening and/or decoupling material must be supported on the housing, so that the thermal expansion, in turn, must have the greatest effect in the moving area of the noise-radiating surface. These compensating movements make it difficult to provide a media-tight transducer design.
Ultrasonic transducers in which a damping element, for example a damping casting, is inserted between the housing and a transducer core which has a piezoceramic transducer element, is described in German Patent Application Nos. DE 10 2008 055 126.0 and DE 10 2008 055 116.3. In general, elastomers, such as O rings or silicone molded parts, are frequently used for decoupling structure-borne noise. In and of themselves, however, these approaches do not usually offer sufficient media resistance for the installation situation, in particular, in motor vehicles. For example, silicone may swell and/or bleed under the influence of moisture and/or hydrocarbons, and gaps may form between the silicone and the surrounding area. As a result, sealing elements, for example in the form of vapor-deposited coatings, for example parylenes or sealing films, are frequently used for sealing ultrasonic transducers. The use of sealing films is also described, for example, in German Patent Application No. DE 10 2008 055 116.3. However, the disadvantage of pure coatings often lies in a poor mechanical cohesion of the coating, in particular under shearing loads, which occur in transition zones between different base materials. For example, the decoupling material may shift in relation to the impedance matching layer or the transducer housing under mechanical or thermal load, so that the coating may crack on this side. When using films, in many cases, thermal and mechanical stresses result in at least in an expansion load on the film, so that the film itself or its adhesion is severely stressed.